Glossary - S
saturation of a perceived color the attribute according to which it appears to exhibit more or less chromatic color judged in proportion to its brightness. In a given set of viewing conditions, and at luminance levels that result in photopic vision, a stimulus of a given chromaticity exhibits approximately constant saturation for all luminances. scoop a floodlight consisting of a lamp in an ellipsoidal or paraboloidal matte reflector, usually in a fixed relationship, though some types permit adjustment of the beam shape. scotopic vision vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m2. See photopic vision. sealed-beam headlamp an integral optical assembly designed for headlighting purposes, identified by the name "Sealed Beam" branded on the lens. sealed-beam lamp A pressed-glass reflector lamp (PAR) that provides a closely controlled beam of light. Note This term is generally applied in transportation lighting (for instance, automotive headlamps and aircraft landing lights) to distinguish sealed-beam lamps from similar devices in which the light source is replaceable within the reflector-lens unit. searchlight a projector designed to produce an approximately parallel beam of light. Note The optical system of a searchlight has an aperture of greater than 20 cm (8 in). secondary standard source† a constant and reproducible light source calibrated directly or indirectly by comparison with a primary standard. This order of standard is also called a reference standard. Note National secondary (reference) standards are maintained at national physical laboratories; laboratory secondary (reference) standards are maintained at other photometric laboratories. A self-calibrated detector can be used as a secondary standard. self-ballasted lamps any arc discharge lamp of which the current-limiting device is an integral part. self-luminous exit sign† an exit sign consisting of phosphor-coated glass tubes filled with a radioactive tritium gas. When the radioactive gas bombards the phosphor, the tube emits light (luminescence) and illuminates the exit legend, typically between 0.2 and 0.7 cd/m2. semi-direct lighting lighting involving luminaires that distribute 60 to 90% of the emitted light downward and the balance upward. semi-indirect lighting lighting involving luminaires that distribute 60 to 90% of the emitted light upward and the balance downward. service period the number of hours per day for which daylighting provides a specified illuminance level. It often is stated as a monthly average. set light in theatrical lighting, the separate illumination of background or scenic elements. shade a screen made of opaque or diffusing material that is designed to prevent a light source from being directly visible at normal angles of view. shielding angle (of a luminaire) the angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible. See cutoff angle (of a luminaire). short-arc lamp an arc lamp in which the distance between the electrodes is small (on the order of 1 to 10 mm). Note This type of lamp (e.g., xenon or mercury) generally has an arc tube containing gas at very high pressure. side-back light illumination from behind the subject in a direction not parallel to a vertical plane through the optical axis of the cameras. See back light. side light lighting from the side to enhance subject modeling and place the subject in depth, apparently separated from the background. side marker lamps lamps indicating the presence of a vehicle when seen from the front and sometimes serving to indicate its width. When seen from the side they can also indicate its length. signaling light a projector used for directing light signals toward a designated target zone. signal shutter a device that modulates a beam of light by mechanical means for the purpose of transmitting intelligence. size threshold the minimum perceptible size of an object. It also is defined as the size that can be detected some specific fraction of the time it is presented to an observer, usually 50%. It usually is measured in minutes of arc. See visual acuity. sky factor the ratio of the illuminance on a horizontal plane at a given point inside a building due to the light received directly from the sky, to the illuminance due to an unobstructed hemisphere of sky of uniform luminance equal to that of the visible sky. sky light† visible radiation from the sun redirected by the atmosphere. sky luminance distribution function for a specified sky condition, the luminance of each direction of the sky relative to the zenith luminance. soft light (1) diffuse illumination that produces soft-edged, poorly defined shadows on the background when an object is placed in its path; (2) a luminaire designed to produce such illumination. solar efficacy the ratio of the solar illuminance constant to the solar irradiance constant. The current accepted value is 94.2 lm/W. solar illuminance constant the solar illuminance at normal incidence on a surface in free space at the earth's mean distance from the sun. The currently accepted value is 127.5 klx (11,850 fc). solar (irradiance) constant† the irradiance, averaging 1353 W/m2 (125.7 W/ft2), from the sun at its mean distance from the earth, 1.5 × 1011 m (92.9 × 106 mi), before modification by the earth's atmosphere. solar radiation simulator a device designed to produce a beam of collimated radiation having a spectrum, flux density, and geometric characteristic similar to those of the sun outside the earth's atmosphere. solid angle, m a measure of that portion of space about a point bounded by a conic surface whose vertex is at the point. It is defined as the ratio of intercepted surface area of a sphere centered on that point to the square of the sphere's radius. It is expressed in steradians. solid angle factor, Q a function of the solid angle m'', given in steradians, subtended by a source and is given by Q = 20.4''m ''+ 1.52''m''0.2 − 0.75. See ''index of sensation. spacing† for roadway lighting, the distance between successive lighting units, measured along the centerline of the street. For interior applications see Chapter 9, Lighting Calculations. spacing-to-mounting-height ratio, S/MHwp† the ratio of the actual distance between luminaire centers to the mounting height above the workplane. Also, an obsolete term that described a characteristic of interior luminaires. See luminaire spacing criterion. special color rendering index, R''i measure of color shift of various standardized special colors, including saturated colors, typical foliage, and Caucasian skin. It also can be defined for other color samples when the spectral reflectance distributions are known. spectral-directional emissivity, ε (λ, ,φ,''T) (of an element of surface of a temperature radiator at a given wavelength and in a given direction) the ratio of its spectral radiance at that wavelength and in the given direction to that of a blackbody at the same temperature and wavelength: spectral hemispherical emissivity, ε(λ, T) (of an element of surface of an opaque temperature radiator that has an optical smooth surface) the ratio of its spectral radiant exitance to that of a blackbody at the same temperature. Note Hemispherical emissivity is frequently called "total" emissivity. However, "total" is ambiguous and should be avoided, since it can also refer to the spectral total (all wavelengths) as well as directional total (all directions). See spectral-total hemispherical emissivity. spectral (spectroscopic) lamp a discharge lamp that emits a significant portion of its radiative power in a line spectrum and that, in combination with filters, can be used to obtain monochromatic radiation. spectral luminous efficacy of radiant flux K(λ) = ①vλ/①eλ the quotient of the luminous flux at a given wavelength by the radiant flux at that wavelength. It is expressed in lm/W. See also radiant flux ''and ''spectral radiant flux. Note This quantity formerly was called the luminosity factor. The reciprocal of the maximum luminous efficacy of radiant flux, that is, the ratio between radiant and luminous flux at the wavelength of maximum luminous efficacy, is sometimes called the mechanical equivalent of light; that is, the ratio between radiant and luminous flux at the wavelength of maximum luminous efficacy. The most probable value is 0.00146 W/lm, corresponding to 683 lm/W as the maximum possible luminous efficacy. For scotopic vision values the maximum luminous efficacy is 1754 scotopic lm/W. spectral luminous efficiency for photopic vision, V(λ)† See values of spectral luminous efficiency for photopic vision. spectral luminous efficiency for scotopic vision, V(λ)† See values of spectral luminous efficiency for scotopic vision. spectral luminous efficiency of radiant flux† the ratio of the luminous efficacy for a given wavelength to the value for the wavelength of maximum luminous efficacy. It is dimensionless. Note This term replaces the previously used terms relative luminosity ''and ''relative luminosity factor. spectral radiant energy, Qλ = dQ/dλ radiant energy per unit wavelength interval, for example, joules per nanometer. Qλ (λ) = d''Q/''dλ at wavelength λ.spectral radiant exitance, MJ., and irradiance, Eλ spectral concentration of radiant exitance, MA = dM/dA, and spectral concentration of irradiance, EA = dE/dA. See radiant flux density at a surface. spectral radiant flux ①λ = d①/dλ radiant flux per unit wavelength interval at wavelength λ, for example, W/nm. spectral radiant intensity, Iλ = d''I/''dλ radiant intensity per unit wavelength interval, for example, W/sr × nm. spectral reflectance of a surface or medium, ro (λ) = ①rλ/①iλ † the ratio of the reflected flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, �λ, about λ. Note The various geometrical aspects of reflectance can each be considered restricted to a specific region of the spectrum and can be so designated by the use of the adjective "spectral." spectral-total directional emissivity, ε ( ,φ,T) (at a point on the surface of a thermal radiator and in a given direction) the ratio of the radiance of the thermal radiator at temperature T at the point and in the given direction, to that of a blackbody at the same temperature, T. where x, y are the coordinates of the point, \theta , φ define the direction. spectral-total hemispherical emissivity, ε (x,y,2π,T) (at a point on the surface of a thermal radiator)† the ratio of the radiant exitance of the thermal radiator at temperature T, at the given point on the surface, to that of a blackbody at the same temperature T: spectral transmittance of a medium τ(λ) = ①tλ/①iλ the ratio of the transmitted flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, �λ, about λ. Note The various geometrical aspects of transmittance can each be considered restricted to a specific region of the spectrum and can be so designated by the addition of the adjective "spectral." spectral tristimulus values† See color-matching functions. spectrophotometer an instrument for measuring the transmittance and reflectance of surfaces and media as a function of wavelength. spectroradiometer an instrument for measuring radiant flux as a function of wavelength. spectrum locus the locus of points representing the colors of the visible spectrum in a chromaticity diagram. specular angle that angle between the perpendicular to the surface and the reflected ray that is numerically equal to the angle of incidence, and that lies in the same plane as the incident ray and the perpendicular, but on the opposite side. specular reflectance† See regular (specular) reflectance. specular reflection† See regular (specular) reflection. specular surface one from which the reflection is predominantly regular. See regular (specular) reflection. speed of light† the speed of all radiant energy, including light, is 2.9979258 × 108 m/s in vacuum (approximately 186,000 mi/s). In all material media the speed is less and varies with the material's index of refraction, which itself varies with wavelength. speed of vision the reciprocal of the duration of the exposure time required for something to be seen. sphere illumination illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center. spherical reduction factor the ratio of the mean spherical luminous intensity to the mean horizontal intensity. Retained for reference or literature search. spotlight any of several different types of luminaires with narrow beam angle designed to illuminate a well-defined area. In motion pictures, generic for Fresnel lens luminaires. Also, a form of floodlight, usually equipped with lenses and reflectors to give a fixed or adjustable narrow beam. standard illuminant A a blackbody at a temperature of 2856 K. It is defined by its relative spectral power distribution over the range from 300 to 830 nm. standard illuminant B a representation of noon sunlight with a correlated color temperature of approximately 4900 K. It is defined by its relative spectral power distribution over the range from 320 to 770 nm. Note It is anticipated that at some future date, that is yet to be decided, illuminant B will be dropped from the list of recommended standard illuminants. standard illuminant C a representation of daylight having a correlated color temperature of approximately 6800 K. It is defined by its relative spectral power distribution over the range from 320 to 770 nm. Note It is anticipated that at some future date, that is yet to be decided, illuminant C will be dropped from the list of recommended standard illuminants. standard illuminant D65 a representation of daylight at a correlated color temperature of approximately 6500 K. It is defined by its relative spectral power distribution over the range from 300 to 830 nm. Note At present, no artificial source for matching this illuminant has been recommended. standard source an electric light source having the same spectral power distribution as a specified standard illuminant. standard source A a tungsten filament lamp operated at a color temperature of 2856 K (International Practical Temperature Scale, 1968) and approximating the relative spectral power distribution of standard illuminant A. standard source B an approximation of standard illuminant B obtained by a combination of source A and a special filter. standard source C an approximation of standard illuminant C obtained by a combination of source A and a special filter. starter a device used in conjunction with a ballast for the purpose of starting an electric-discharge lamp. state of chromatic adaptation the condition of the chromatic properties of the visual system at a specified moment as a result of exposure to the totality of colors of the visual field currently and in the past. Stefan-Boltzmann law the statement that the radiant exitance or radiance of a blackbody radiator is proportional to the fourth power of its absolute temperature; that is, Note The currently recommended value of σ is 5.67032 × 10−8 (W × m−2 × K−4) and that of σL is 1.80492 × 10−8 (W × m−2 × sr−1 × K−4) stencil face exit sign† a transilluminated sign where either the exit legend or the background are opaque. Usually the exit legend is translucent and the background is die cut from an opaque medium such as plastic or metal. steradian, sr (unit of solid angle)† the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius. stilb a cgs (cm-gram-second) unit of luminance. One stilb equals 1 cd/cm2. The use of this term is deprecated. Stiles-Crawford effect the reduced luminous efficiency of rays entering the peripheral portion of the pupil of the eye. This effect applies only to cones and not to rods. Hence, there is no Stiles-Crawford effect in scotopic vision. stop lamp a device giving a steady warning light to the rear of a vehicle or train of vehicles, to indicate the intention of the operator to diminish speed or to stop. stray light (in the eye) light from a source that is scattered onto parts of the retina lying outside the retinal image of the source. street lighting luminaire a complete lighting device consisting of a light source and ballast, where appropriate, together with its direct appurtenances such as globe, reflector, refractor, housing, and such support as is integral with the housing. The pole, post, or bracket is not considered part of the luminaire. Note Modern street lighting luminaires contain the ballasts for high-intensity discharge lamps where such lamps are used; a photocontrol can be mounted on the luminaire. street lighting unit the assembly of a pole or lamp post with a bracket and a luminaire. striplight (theatrical) once an open trough reflector containing a series of lamps; now usually a compartmentalized luminaire with each compartment containing a lamp, reflector, and color frame holder, wired in rotation in three or four circuits and used as borderlights, footlights, or cyclorama lighting from above or below. Often in short 0.9- to 2.4-m to 8-ft portable sections. stroboscopic lamp (strobe light) a flash tube designed for repetitive flashing. subjective brightness the subjective attribute of any light sensation giving rise to the perception of luminous magnitude, including the whole scale of qualities of being bright, light, brilliant, dim, or dark. See saturation of a perceived color. Note The term brightness often is used when referring to the measurable luminance. While the context usually makes it clear as to which meaning is intended, the term luminance ''should be used for the photometric quantity, thus reserving brightness for the subjective sensation. sun bearing the angle measured in the plane of the horizon between a vertical plane at a right angle to the window wall and the position of this plane after it has been rotated to contain the sun. sunburn inflammation with reddening (erythema) of the skin, of variable degree, caused by exposure to direct or diffuse solar radiation or artificial optical radiation. sun lamp an ultraviolet lamp that radiates a significant portion of its radiative power in the UV-B band (280 to 315 nm). sunlight direct visible radiation from the sun. suntan a darkening of the skin due to an increase of melanin pigmentation above constitutive level and induced by UV radiation. supplementary lighting lighting used to provide an additional quantity and quality of illumination that cannot readily be obtained by a general lighting system and that supplements the general lighting level, usually for specific work requirements. supplementary standard illuminant D55 a representation of a phase of daylight at a correlated color temperature of approximately 5500 K. supplementary standard illuminant D75 a representation of a phase of daylight at a correlated color temperature of approximately 7500 K. surface-mounted luminaire a luminaire that is mounted directly on a ceiling. suspended (pendant) luminaire a luminaire that is hung from a ceiling by supports. switch start fluorescent lamp† See preheat ''(switch start) fluorescent lamp. Systeme Internationale (SI)† a measurement system used throughout the world, commonly referred to as the metric system. Public Law 100-418 designated the metric system as the preferred system of weights and measures for the United States. T